For several decades, consumer blocks of cheese have been packaged in form-fill-and-seal packaging machines. The most common form-fill-and-seal packaging machine for the packaging of cheese is the Hayssen® RT horizontal form-fill-and-seal packaging machine. The Hayssen® RT packaging machine forwards the film by pulling on the downstream portion of the film with hot seal bars. See U.S. Pat. No. 3,943,683, which discloses a Hayssen® RT packaging apparatus.
In order to be operable on the Hayssen® RT machine, the packaging film must have good strength when hot, i.e., good strength at a relatively high yield point, or the film will fail (break) during the transverse sealing step. Most of the films which have been used on the Hayssen® RT packaging machine in the past have been laminated barrier films. These films have been prepared by laminating together an oxygen-barrier film with at least one heat-resistant film. These films were, of course, separately extruded. The oxygen-barrier film has a layer which is a barrier to the transmission of atmospheric oxygen, to extend the shelf-life of the cheese being packaged. The heat-resistant film has a layer providing a high yield point, to allow the laminate to undergo processing on the Hayssen® RT packaging machine, without the film yielding under the combination of heat and tension encountered during transverse sealing. Production of the film using a lamination-based process has been carried out because the extrusion conditions for the biaxially-oriented crystalline polyester and/or polypropylene used in the heat-resistant film are not compatible with the extrusion conditions for the barrier film. Moreover, the heat-resistant film (films) has been made using a polyester and/or polypropylene which has been biaxially oriented in the solid state and thereafter heatset,which is not necessary for the barrier film.
Another film which has been used for cheese packaging on the Hayssen® RT machine is a fully coextruded film which is oriented in the solid state and thereafter heatset. This coextruded film contains an oxygen-barrier layer, as well as a temperature-resistant layer. The extrudate must be biaxially-oriented and heatset in order for the crystalline polyamide to have the desired heat-resistant properties and tensile modulus for the film to be operable on the Hayssen® RT packaging machine. It would be desirable to provide a film suitable for the packaging of cheese on the Hayssen® RT packaging machine, without having to either laminate two films together and without having to orient a film in the solid state.